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mid-ocean ridge, while the Pacific is shrinking, consumed by the zones that surround it, the famous ring of fire. The next “If you simply run plate tectonics forward in time, you would see the Pacific close and the Atlantic open,” says Mark Behn at Woods Hole Oceanographic Institution in Massachusetts. In about 250 million years a new supercontinent, Novopangaea, would form on the opposite side of from the original “We’re still struggling to The formation of a new mega land mass is just a understand the rules of matter of geological time. But what will it look like, plate tectonics” asks Stephen Battersby , as the and crunch in around northbound . But it may not be that simple. “You can get to 50 million years by projecting present-day SIA is torn in two. The Atlantic and earthquakes and feeds volcanoes. motions,” says Christopher Scotese at Pacific oceans are swallowed. Where once Magnetic signals recorded in sea-floor Northwestern University in Evanston, Illinois. Athere were beaches, great mountain rocks, and chemical traces from the roots “To go any further, you have to understand ranges judder into the skies, fusing together of ancient mountain ranges, tell us how why plates are moving, what are the driving a scatter of separate land masses into one continental drift has changed the face of forces, what are the rules of plate tectonics.” mighty new supercontinent. Call it… . Earth. They point clearly to a time 180 million That’s something we are still struggling to do. That’s what João Duarte calls it, anyway. years ago when all today’s were Since 1982, Scotese has been making maps A geoscientist at the University of Lisbon, stuck together in one vast land mass centred of past and future Earth using various rules Portugal, he has his own distinct vision of how roughly where present-day is: the of thumb. The most important rule is widely Earth may look 250 million years from now. supercontinent Pangaea, from the Ancient accepted: plate tectonics is driven mainly by He joins a band of fortune tellers gazing into Greek for “all of Earth”. the pull of sinking slabs at subduction zones, the distant future, all with different ideas with a smaller push from new rock forming about how and where the next supercontinent at mid-ocean ridges. Work out the layout of will form, and what cataclysms might strike Cycling continents subduction zones and ridges at any time, along the way. We know that Pangaea came together about and you can begin to see how the continents The answer will determine Earth’s future 330 million years ago. Before that, some should be tugged and nudged around. climate and prospects for sustaining life. consider a relatively short-lived gathering of But three kinds of cataclysmic event can But getting it right requires grappling with a continents near the South Pole to be another change the course of this smooth voyage. machine whose workings we still understand supercontinent, named or Greater A subduction zone can swallow a spreading only imperfectly: that of plate tectonics. . Another supercontinent, , ocean ridge, as is happening today off North Earth’s surface is clad in rigid rock plates – probably dominated the planet between about America’s west coast, where the Juan de Fuca together called the lithosphere – formed of 1.2 billion and 700 million years ago. And ridge is slowly being consumed. Or a pair of surface crustal rock laminated on to hard about 2 billion years back it is thought there unsinkable continents can collide, snuffing cold mantle rocks. Given their rigidity, it is was another, known as Nuna or Columbia. out subduction in between and forcing a great surprising that these plates don’t simply lock What has been will be. “For more than mountain range, as India and have together, unmoving. And indeed, until about 20 years we have recognised that Pangaea was done to build the Himalayas. 50 years ago geologists thought that Earth’s just the latest in a series of ,” The third possible cataclysm is much harder land masses were fixed, despite German says Brendan Murphy at St Francis Xavier to fathom or predict. “The fundamental geophysicist Alfred Wegener having proposed University in Antigonish, Canada. “That implies question is how you start new subduction the idea of continental drift in 1915. there will be another one in the future.” zones,” says Behn. The creation and destruction of ocean What’s unclear is how that vast land mass This has to happen somehow, or all existing basins makes plate motion possible. Plates will form. One model simply projects what’s subduction zones would eventually be killed move apart at mid-ocean ridges, where molten happening today into the future. The great by continental collisions, and place tectonics rock rises and cools to form hard, dense basalt. split that broke Pangaea apart is still growing, would cease altogether. In 2008, Behn and They move together at subduction zones, and the two biggest land masses on Earth, his colleague Paul Silver suggested that if a where old ocean lithosphere plunges under Africa-Eurasia on the one hand and the supercontinent forms simply by closing the a neighbouring plate. As it penetrates the Americas on the other, are on the move. The Pacific, destroying all the main subduction

BRETT RYDER warmer, softer mantle beneath, it causes Atlantic is spreading as new rock wells up at its zones around it, plate tectonics could shut >

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171014_F_SuperContinent.indd 34-35 16/10/2017 17:56 down for a long time. They suggest something from mid-ocean ridges, meets continental In the 1980s, Scotese suggested that stress that the future is uncertain, and that similar may have happened in the past. crust. The oceanic lithosphere has had time subduction is catching. While it is difficult their own model is just one option (although But even if that is true, evidence that plate to cool since formation and become denser to break old ocean lithosphere from scratch, of course the most likely one). Whoever is tectonics has gone on for almost all of Earth’s than the rock beneath, so it wants to sink. “a nick will localise stresses and tear more right, our distant cousins will have to adapt LINDSAY LOU/GETTY history, with many cycles of supercontinent But it can’t. Old, cold lithosphere rocks are easily”, he says. to a strangely shaped , and will in turn formation and destruction, indicates that new hard to crack. Even the weight of kilometres- There are already two small subduction be shaped by it. “The whole Earth system is subduction must start eventually, somewhere. deep river sediments washed on to the passive zones in the western Atlantic: the Lesser ultimately controlled by plate tectonics,” says The most likely spot is at passive margins, margin from the continents isn’t enough on Antilles volcanic arc near the Caribbean, and Scotese. As continents move through different for example on the Atlantic coasts of , its own. The weakening effect of water seeping the Scotia arc in the far south off Tierra del climate zones, they cause new problems for Africa and the Americas. These are places into the rocks may help, but probably not Fuego. These both look as though they have existing life forms and create opportunities where old oceanic lithosphere, spreading out enough to crack those passive margins. sneaked in from old subduction zones in the for others. Extreme volcanism can also cause Pacific ocean. Scotese suggests that eventually or at least contribute to mass extinctions, as they will spread south and north, joining up in the vast outpouring of lava that formed the our futures of Earth to form a long subduction zone up the east Deccan Traps in India about 65 million years coast of the Americas. In his projection, this ago. This changed the global climate and may Depending on how plate tectonics plays out, four very di erent supercontinents could form will eat the Atlantic’s mid-ocean ridge about have put the dinosaurs under serious stress, Africa Australia Antarctic Eurasia New land 100 million years from now, and the Atlantic before a meteorite provided the knockout. will start to close again. After 250 million angaea roima Aurica years, the Americas will have collided with an already merged Africa and Eurasia – as will Driving forces Australia and most of – to form Climate may differ wildly between different what Scotese calls Pangaea Proxima. supercontinent scenarios, affecting Earth’s Others have come to a similar conclusion by New rock welling up at ocean ridges Atlantic’s eastern margin, off the coast of habitability. , near the North Pole, looking at the geological record, which shows helps to drive plate tectonics Portugal, where forces generated by the might gather a massive ice cap. Novopangaea that oceans periodically open and close in remnants of an ancient subduction in the could be similar to original Pangaea, which something known as the Wilson cycle. In the Amasia relatively straggly, with the Americas Mediterranean are helping to create new may have seen extremes of weather, with a 1980s, Thomas Worsley and Damian Nance forming a huge promontory and Antarctica faults in the ocean floor. vast interior and a seasonal “mega- million years million years at Ohio University suggested that the next remaining aloof, unlike the compact form of In Duarte’s model of Earth’s future, monsoon”. Or if plate tectonics were to shut supercontinent might form more or less in the original Pangaea and the other imagined published last year, subduction will spread down for a while, that would radically affect the the way it split up, by closing the Atlantic. supercontinents of the future. along both sides of the Atlantic within a few atmosphere. Volcanoes would cease to pump Duarte thinks all these models have tens of millions of years, and the ocean will out carbon dioxide, and the planet might enter problems. Amasia and Novopangaea would begin to close. But the Pacific will keep on a severe ice age. We could model the climate Old and crusty both be surrounded by large areas of ocean closing, too, meaning something else has on each hypothetical supercontinent – “but In 2012, Ross Mitchell, then at Yale University, crust that is more than 400 million years old, to give. That something is Asia. A rift cuts that is building a house of cards then building and his group mapped out a third route. The which he finds implausible. In 2008, Dwight across the , from the balconies on your house of cards,” says Scotese. shifting of mass associated with the formation Bradley at the US Geological Survey in up to the Arctic, as the Himalayan Plateau Earth’s future depends on what forces really of a supercontinent affects Earth’s rotation, Anchorage, Alaska, looked at rocks around drive the motion of the continents, and that million years million years changing its spin axis relative to the solid body ancient passive margins and found the oldest “Our distant cousins will remains the real unknown. The only way we In around 100 million years, subduction spreading Subduction starts on both sides of the Atlantic, and of the planet. By looking at the orientation of were about 180 million years old on average, can cut through the profusion of possibilities along the western side of the Atlantic causes it to both Atlantic and Pacic close. Eurasia splits, its magnetic crystals in rocks that cooled around and much less than that in recent ages. Hardly is to make plate tectonics more quantitative, start closing. North America ends up fused with the western half moving westwards with Africa, and its be shaped by a strangely west coast of Africa, with South America swinging eastern half migrating eastwards. It scoops up the time that different supercontinents existed, any lasted 400 million years. Duarte thinks says Duarte. “To make substantial progress round to end up at the south of a new supercontinent Australia to form a new supercontinent, centred the team showed that Rodinia formed about this is no coincidence. “Somehow plates in shaped world” we need more observations of the Earth’s centred on the present-day Atlantic where the Pacic is now 90 degrees in latitude away from the position Atlantic-type oceans may have to start interior,” he says. “For example, there are of Nuna, and Pangaea about 90 degrees subducting after about 200 million years,” collapses under its own weight. A new ocean barely any permanent seismometers on the Amasia Noopangaea from Rodinia. Mitchell and his colleagues he says. opens up, and the eventual outcome is a new bottom of the oceans.” We could also use predict that the same thing will happen again, Scotese’s Pangaea Proxima does not have supercontinent with the two halves of Asia on neutrinos from the sun to probe the planet’s meaning the next supercontinent should the old-crust problem: the Pacific could in the outside and American and Australia at its interior, he suggests. Certainly more powerful form somewhere near the North Pole, as Asia theory stay open for many hundreds of core – hence Aurica. models that can capture geochemical processes and North America crunch together. They call millions of years with new crust constantly Nice try, says Scotese. “Trying to close the on scales large and small are needed. the result Amasia. being generated and destroyed. But Duarte Atlantic and Pacific – I think that’s original.” So trying to gaze into Earth’s distant future Some support for this view came in 2016, considers this improbable too, because ridges But suggesting a subduction zone on the may help us to understand the inner workings when Masaki Yoshida at the Japan Agency such as the Juan de Fuca are already being eastern as well as the western side of the of our complex and opaque world today. But for Marine-Earth Science and Technology in subducted. “It may not be very likely that new Atlantic actually makes things more difficult, important though that is, it’s not the real million years million years Yokosuka published a numerical simulation ones form in the middle of the oceanic plates he says, because that could preserve the mid- motivation for sketching out the next The Atlantic continues to open but widens more Current tectonic movements continue, with the of mantle motion. It shows continents where they are cold and strong,” he says. Atlantic ridge. “To close the ocean you have to supercontinent, 10 million generations at the south. Africa moves west and Australia Atlantic widening and the Pacic consumed by converging near the North Pole, guided there Duarte agrees with Scotese that subduction subduct the ridge – but if you have subduction removed. “It’s for fun,” says Duarte. n subduction. South America swings westwards and moves north; meanwhile South America pivots partly by plumes of hot, rising mantle rocks may spread like a virus, a process he calls all around the ocean, the ridge can stay in the and ends up with its west coast fused to North northwards, scooping up Antartica and Australia; America’s east coast. Antarctica remains aloof Africa rotates anticlockwise, taking western Europe that help to keep the Pacific open in the invasion. He has found evidence that middle and supply crust to both sides.” Stephen Battersby is a consultant for New Scientist from this straggly new supercontinent with it, its current south ending up fused with Arabia south while it closes in the north. This makes subduction is beginning to invade the The proponents of each idea are keen to based in London

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